An integrated circuit contains various semiconductor devices and a plurality of conducting metal paths that provide electrical power to the semiconductor devices and allow these semiconductor devices to share and exchange information. Within the integrated circuit, metal layers are stacked on top of one another using intermetal and interlayer dielectric layers that insulate the metal layers from each other.
Normally, each metal layer must form an electrical contact to at least one additional metal layer. Such electrical contact is achieved by etching a feature (i.e., a via) in the interlayer dielectric that separates the metal layers, and filling the resulting via with a metal to create an interconnect. Metal layers typically occupy etched pathways in the interlayer dielectric. A “via” normally refers to any feature such as a hole, line or other similar feature formed within a dielectric layer that provides an electrical connection through the dielectric layer to a conductive layer underlying the dielectric layer. Similarly, metal layers connecting two or more vias are normally referred to as trenches.
The use of copper (Cu) metal in multilayer metallization schemes for manufacturing integrated circuits creates problems due to high mobility of Cu atoms in dielectrics, such as SiO2, and Cu atoms may create electrical defects in Si. Thus, Cu metal layers, Cu filled trenches, and Cu filled vias are normally encapsulated with a barrier material to prevent Cu atoms from diffusing into the dielectrics and Si. Barrier layers are normally deposited on trench and via sidewalls and bottoms prior to Cu seed deposition, and may include materials that are preferably non-reactive and immiscible in Cu, provide good adhesion to the dielectrics and can offer low electrical resistivity.
An increase in device performance is normally accompanied by a decrease in device area or an increase in device density. An increase in device density requires a decrease in via dimensions used to form interconnects, including a larger aspect ratio (i.e., depth to width ratio). As via dimensions decrease and aspect ratios increase, it becomes increasingly more challenging to form diffusion barrier layers with adequate thickness on the sidewalls of the vias, while also providing enough volume for the metal layer in the via. In addition, as via and trench dimensions decrease and the thicknesses of the layers in the vias and trenches decrease, the material properties of the layers and the layer interfaces become increasingly more important. In particular, the processes forming those layers need to be carefully integrated into a manufacturing process sequence where good control is maintained for all the steps of the process sequence.
The problems associated with the use of Cu metal in increasingly smaller features in a substrate will require replacing the Cu metal with other low-resistivity metals. New methods for void-free metal filling of these increasingly smaller features is required.